The present invention relates generally to a semiconductor device and, more particularly, to a structure of the semiconductor device of a resin sealing type known as a chip scale package in which leads are provided within a size of a semiconductor chip.
With advancements of an IC card (an integrated circuit) and a memory card, semiconductor devices incorporated into these cards have been required to be reduced both in thickness and in size. A chip scale package has been developed to meet those demands, and is contrived to downsize the whole by accommodating leads connected to a circuit forming surface of a semiconductor chip within a size of the chip as well as to reduce the thickness by forming a sealing resin layer in such a form that a reverse surface to the circuit forming surface is exposed.
FIGS. 6a and 6b are structural views illustrating a resin sealing type semiconductor device of the prior art chip scale package. FIG. 6a is a perspective view showing an external appearance thereof. FIG. 6b is an enlarged sectional view taken along the line A--A in FIG. 6a. This semiconductor device includes a semiconductor chip 1, and a plurality of electrodes 2 are formed at a central portion of a circuit forming surface 1a serving as one surface thereof.
A lead 4 is a thin metal plate crooked at two points. The lead 4 has a electrode connecting portion 4a bonded to the circuit forming surface 1a at the central portion of the circuit forming surface 1a through an adhesive tape 3 exhibiting an insulating property, an external connecting portion 4b formed with a step substantially in parallel to the connecting portion 4a, and an inclined middle portion 4c for connecting the electrode connecting portion 4a to the external connecting portion 4b. The electrode connecting portion 4a of the lead 4 is electrically connected to the electrode 2 via a metal wire 5. The external connecting portion 4b is disposed at a predetermined interval with respect to the circuit forming surface 1a.
The circuit forming surface 1a of the semiconductor chip 1 and the peripheral side surface are sealed with a sealing resin layer 6 formed by molding in a state where only the external connecting portion 4b of the lead 4 is exposed. On the other hand, a reverse surface 1b opposite to the circuit forming surface 1a of the semiconductor chip 1 is not covered with the sealing resin layer 6 and is in a state of being exposed.
The thus constructed semiconductor device is manufactured by, e.g., the following processes.
To start with, the lead 4 of a molded lead frame is bonded to the circuit forming surface 1a of the semiconductor chip 1 by use of the adhesive tape 3, and thereafter the electrode connecting portion 4a of this lead 4 is electrically connected to the electrode 2 on the semiconductor chip 1 through bonding of the metal wire 5. The electrode connecting portion 4a is silver-plated for wire bonding when manufacturing the lead frame.
Next, the semiconductor chip 1 is mounted into a bottom mold for molding in such a state that the lead frame is bonded. At this time, the semiconductor chip 1 is mounted so that the reverse surface 1b of the semiconductor chip 1 comes into contact with a bottom portion of the bottom mold for molding while a frame member of the lead frame is positioned in a predetermined place in a frame of the bottom mold. Further, the bottom mold into which the semiconductor chip 1 is mounted, is covered with a top mold paired with this bottom mold, and a liquid mold resin is poured into the molds from a gate.
After the mold resin has been hardened, the semiconductor chip molded by the sealing resin layer 6 is taken out of the molds, and an unnecessary portion of the lead frame is cut off. Then, the external connecting portion 4b of the lead 4 is silver-plated for soldering, thus completing the semiconductor device.
When the thus constructed semiconductor device is packaged on the printed-circuit board, cream solder is coated by screen printing etc over a foot print on a parts mounting surface of the printed-circuit board. Then, the semiconductor device is mounted so that the external connecting portion 4b of the lead 4 comes into contact with an upper area of the cream solder. The mounted semiconductor device is put into a reflow device and heated at approximately 300.degree. C. enough to melt the cream solder, thereby soldering the semiconductor device to the printed-circuit board.
There arise, however, mainly two problems, as shown in FIGS. 7a and 7b, inherent in the conventional resin sealing type semiconductor device. A first problem is that the mold resin is forced out onto a part of the external connecting portion 4b of the lead 4 of the semiconductor device when in the molding process, resulting in a production of a so-called resin burr 7 illustrated in a perspective view of FIG. 7a. When the lead frame is mounted into the molds, the external connecting portion 4b of the lead 4 comes into a state of floating within a cavity, and there is no contrivance to press it against an internal upper surface of the top mold. Therefore, an adhesion to the top mold is weak, and there might be a high possibility in which the mold resin poured into the molds permeates between the external connecting portion 4b and the internal upper surface of the top mold, thereby producing the resin burr 7. The resin burr 7 hinders the semiconductor device from being soldered to the printed-circuit board, and hence there is a necessity for removing the resin burr 7 by a contrivance such as, e.g., flushing the burrs away by high-pressure water before being packaged on the printed-circuit board. This conduces to a problem of increasing the number of processes therefor.
Moreover, concomitantly with the first problem, the prior art resin sealing type semiconductor device has a problem of requiring two plating processes with respect to the lead 4 before and after forming the sealing resin layer 6. More specifically, if the resin burr 7 is produced, as described above, the process of removing this burr is needed, and hence the silver plating for soldering, which is effected on the external connecting portion 4b, is required to be executed after the process of removing the resin burr 7. While on the other hand, it is required that the silver plating for wire bonding with respect to the electrode connecting portion 4a be executed when manufacturing the lead frame.
A second problem is that a crack 11 tends to be caused in the solder 10 for a connection to a foot print 9 on a printed-circuit board 8 as illustrated in FIG. 7b showing a packaged state of the semiconductor device on the printed-circuit board. The crack is derived from a brittle fracture in the solder 10, which is caused in such a process that if used in an environment where the temperature fluctuates wildly after being packaged on the printed-circuit board, or when cooled down to a normal temperature after being heated by the reflow device, a stress is applied upon the solder 10 due to a difference in terms of thermal expansion coefficient between the sealing resin layer 6 and the printed-circuit board 8. If the crack 11 is produced in the solder 10, there arises a problem in which a mechanical connecting strength between the printed-circuit board 8 and the semiconductor device decreases, and the electrical connection becomes unstable.
Accordingly, it is a primary object of the present invention, which was contrived in view of the above problems inherent in the prior art, to provide a resin sealing type semiconductor device capable of making a resin burr hard to occur when in a molding process and preventing cracks from being produced in solder.